This invention relates to an anti-lock control method for preventing a wheel lock when braking a running vehicle.
Generally, in an anti-lock control apparatus for a vehicle, for the purpose of securing the steerability and the running stability of the vehicle and of reducing the braking distance when braking the vehicle, a control mode of brake hydraulic pressure is determined in accordance with an electric signal representative of a wheel speed (rotational speed) detected by a wheel speed sensor, and a hold valve in the form of a normally-open solenoid valve and a decay valve in the form of a normally-closed solenoid valve are opened and closed, so that the brake hydraulic pressure is controlled by a control unit, including a microcomputer, to be increased (pressurized), held and decreased.
FIG. 1 is a block diagram of a conventional 3-channel anti-lock control apparatus for performing such anti-lock control. Outputs of wheel speed sensors 1 to 4 are fed respectively to arithmetic circuits 5 to 8, and are calculated to produce signals respectively representing wheel speeds Vw1 to Vw4. The left front wheel speed Vw1 and the right front wheel speed Vw2 are directly fed, as first and second system speeds Vs1 and Vs2, to first and second control logic circuits 9 and 10, respectively. A lower one out of the left rear wheel speed Vw3 and the right rear wheel speed Vw4 is selected by a low-select circuit 11, and is fed, as a third system speed Vs3, to a third control logic circuit 12.
The signals representing the wheel speed Vw1 to Vw4, respectively, are fed to an estimated vehicle speed calculating circuit 13 comprising a high-select circuit 14 and a filter circuit 15. In this calculating circuit 13, the highest wheel speed VwH among the four wheel speeds Vw1 to Vw4 is selected (four-wheel select-high) by the high-select circuit 14, and a speed whose follow-up limit relative to the highest wheel speed VwH is restricted to a range, for example, of .+-.1G is obtained by the filter circuit 15, and is fed, as an estimated vehicle speed Vv approximating an actual vehicle speed, to the control logic circuits 9, 10 and 12. In the control logic circuits 9, 10 and 12, the system speeds Vs1 to Vs3 are used respectively as control vehicle speeds (herein after referred to as "system speed Vs") of the channels, and the ON-OFF control of the hold valve HV and the decay valve DV is effected by using this system speed Vs and the estimated vehicle speed Vv as a reference.
In the above anti-lock control method, the estimated vehicle speed Vv is an important factor for the reference speed for the anti-lock control. Therefore, in the case where the four wheel speeds Vw1 to Vw4 are reduced at the same time by braking of the vehicle, the estimated vehicle speed Vv calculated based on the highest wheel speed VwH among the four wheel speeds is also reduced as shown in FIG. 2, so that the difference between the estimated vehicle speed Vv and the actual vehicle speed becomes greater. Due to this, there has been encountered a problem that when the control is effected based on the estimated vehicle speed Vv, the control can not be carried out in a stable manner.